1. >cd ~/.mozilla/firefox/*default*/Cache
2. go to youtube, and watch the video you want to download
3. >file */*/* | grep -i "media" | perl -ne '@f=split(/\:/,$_); system("ls -l ".$f);' | sort -k7 | sort -n -k5 | tail -5
4. Look at the last few lines, the very last line is probably your video. Copy it to another directory, and give it a humane name.